In recent years, a vehicle is equipped with a remote control system which performs wireless communication between an on-vehicle control unit and an electronic key held by a user so as to open and close doors of the vehicle in a non-contact manner. The electronic key which is carried outside the vehicle by the user is equipped with an inner casing in which an electronic circuit for performing wireless communication with the control unit is installed, and a battery for driving the electronic circuit. In order to prevent a failure of the electronic circuit inside the electronic key, a waterproof rubber is provided inside the casing of the electronic key for the waterproof and dustproof purposes. The waterproof rubber covers an upper surface side of the inner casing and is coupled to an upper casing. The battery is provided at the lower surface side of the inner casing (for example, see PTL 1).